A burglar already in prison for 13 years has had six more added to his sentence for stabbing a fellow prisoner in the neck.

Dean Michael Vincent, 49, had been found guilty by a jury of wounding with intent to cause grievous bodily harm to Shane Stoneham.

Stoneham is serving preventive detention for sexual offences against women and had been caught in bed with a 14-year-old girl while he was under a 24 hour watch in the community.

Wellington District Court judge Bruce Davidson said Vincent loathed and despised Stoneham and after an incident in the exercise yard between the two where Vincent thought Stoneham was kicking balls at him, they were separated.

For several days Vincent worked on making a knife from a toothbrush and metal clips from a ring binder. He then walked up to Stoneham in a shared area and stabbed him four times in the neck.

Stoneham received cuts, but his injuries did not turn out to be serious. The judge said it was cruel in the extreme and Vincent used a potentially lethal weapon.

He said Vincent had an irrational hatred of Stoneham and his beliefs were so firmly held that he justified the stabbing to himself.

The judge added the six years jail term for the stabbing on top of the time Vincent was already serving.

Vincent, who acted for himself at the trial and sentencing, said he now accepted Stoneham had suffered but said it was not without aggravation from Stoneham.

He said he had been working hard toward his release and knew he was going to get another jail term.